Digitizing systems including cordless electric field, and also cordless magnetic field, coupled pointing device (including pens, cursors, mouses, etc.) are well known. Commonly assigned U.S. Pat. Nos. 4,859,814, 4,672,154, and 4,748,295, incorporated herein by reference, are generally indicative of the state of the art. Two types of digitizers, those with "electric field coupling" between the pointer and the digitizing grid and those with "magnetic field coupling" between the pointer and the digitizing grid have been widely used. It is known that such digitizing systems frequently must operate in environments with high electrical noise over a broad frequency spectrum. The major sources of such electrical noise are the fundamental and harmonic frequencies produced by a wide variety of common video monitors. In an office environment, computer monitors are the major source of electrical interference which would interfere with digitizing tablets, but there also are other noise sources, sources such as fluorescent lights and switching power supplies.
These noise sources all generate electric fields and electromagnetic fields that can interfere with the signal to be transmitted from the pointing device to the grid of a digitizer tablet. This is a particular problem in a cordless system. In general the spectrum of the fundamental frequencies of this interference is mostly under 100 KHz and the harmonics are mostly above KHz. In the frequency range from 300 KHz to 550 KHz most of the interference is harmonic in nature and subsequently the levels of interference are significantly less than at frequencies below 100 KHz. Operation in the frequency range from 300 KHz to 550 KHz is possible with a magnetic tablet or digitizer with the added benefit of increased transmission efficiency from the pointing device to the grid conductor due to the improved magnetic coupling at these higher frequencies. However, care must be taken in the design of the grid configuration to ensure that the self-resonance frequency of the grid is sufficiently above the operating frequency range so as to not to result in significant cross coupling between grid conductors so as to result in increased current flow that causes one line or loop to transmit a signal to other lines or loops. Major cross coupling of signals between grid conductors causes unpredictable errors in determining the pointing device position, and such errors are not amenable to usual correction techniques.
In contrast, operation of electric field coupled tablets or digitizers at frequencies much above 100 KHz is difficult or impractical due to the increased pointing device drive power required and the greater shunting effect that grid capacitance has on the "high source impedance"characteristic of an electric field induced grid signal.
Commonly assigned U.S. Pat. No. 4,859,814, entitled "NOISE CANCELLATION IN DIGITIZING SYSTEMS AND METHOD issued on Aug. 22, 1989, by Sciacero, et al. incorporated herein by reference, discloses use of a differential sensing technique in an electric field coupled system to cancel ambient noise.
It is generally accepted by those skilled in the art that digitizing systems based on magnetic field coupling between the pointing device and the digitizing grid are more "robust" than electric field coupling systems in the sense that they are less affected by the nearness of the user's hand, moisture, partial conductivity of materials put on the tablet surface such as certain inks or pencil leads, and conductive or dielectric effects of drawing instruments and other items in close proximity to the digitizing surface, and are generally more accurate and less affected by external environmental effects than electric field coupled digitizing systems.
In the past, magnetic field coupled tablets or digitizers have operated at carrier frequencies less than 100 KHz for several reasons. First, the grid configurations of prior magnetic field coupled tablets, especially those with repeating grid patterns, have had a low self-resonance frequency, and operation at any higher frequency results in uncorrectable positional errors. Second, the gain bandwidth product of the tablet carrier amplification/filtering stages of prior magnetic field coupled tablets has been limited due to the use of low frequency and low cost operational amplifier integrated circuits in these stages instead of discrete video type circuits. Third, the power consumed by the pointing devices of prior magnetic field coupled tablets has been less important because the pointing devices have been connected by a cable to the tablet, so the drive power could be as high as necessary to achieve an adequate induced grid signal-to-interference ratio.
In one prior art system, a cordless magnetic design utilizes a passive pointing device design. In this system a magnetic field is transmitted from the surface or grid and coupled into a high Q tuned circuit in the pointing device. The magnetic field then is turned off and the residual magnetic energy in the tuned circuit is received by the grid and used to resolve position of the pointing device until the residual energy is spent. This system has the advantage that the pointing device is essentially a passive device with minimum circuitry and without a need for additional power. However, this system requires periodic wait intervals when no detection of the pointing device signal is taking place and instead the "passive" pointing device is being "excited" by a magnetic signal being radiated from the tablet grid. This also means that the duty cycle of the received signal is greatly reduced, reducing the overall signal-to-noise ratio over time.
The above-described system also requires that the signal induced onto the grid by the pointing device be subjected to a two-way path loss. This two way loss generally leads to the requirement of more overall power for operation than the one-way loss to which the radiated signal of the present invention is subjected. While reducing circuitry in the pointing device, the foregoing technique requires added complexity to the grid structure since many lines or coils must be switched and driven with high power. This is particularly relevant in tablets used in portable or battery operated applications. Also, the foregoing approach requires that the grid be able to transmit as well as receive signals in order to operate. This requires that the grid conductors be of low resistance or impedance in order to obtain sufficient current and resulting magnetic field intensity to effectively transmit to the transducer coil. This leads to the requirement that the grid utilize low resistance wire or printed circuit elements in order to achieve sufficient power transmission. That prevents or limits the use of low cost and low-weight printed grids, such as those having silver ink printed on mylar film. The foregoing technique results in use of more power, generally lower proximity height for the pointing device, and reduced signal-to-noise ratio and increased susceptibility to noise, jitter, etc.